This invention relates to microwave identification systems comprising a transmitter/receiver provided with an antenna system, a plurality of responders provided with an antenna system, the communication between transmitter/receiver and responders taking place by means of electromagnetic waves in the microwave range. The invention further relates to a responder for a microwave identification system.
Microwave identification systems are known in various designs. Reference is made to the following U.S. Patents: U.S. patent application No. 4,816,839, U.S. patent application No. 4,853,705, U.S. patent application No. 4,782,345, U.S. patent application No. 4,242,661.
A drawback of the known systems is that all of them utilize antenna systems that have a low to very low antenna amplification factor.
U.S. Pat. No. 4,853,705 in fact indicates that array-like antenna systems are not eligible for use precisely because of their-higher antenna amplification factors and inherent narrow beam widths.
In an identification system, an electronic interrogation unit can read out a code stored in the transponder and thereby identify the transponder.
The interrogation unit of a microwave identification system comprises a transmitter with a transmitting antenna which radiates an electromagnetic wave in the direction of the transponder and a receiver which receives the signal modulated by the responder and identifies the code of the responder.
The responder comprises an antenna system, a supply device for generating a DC voltage, a modulation device capable of modulating an electromagnetic wave and a chip with a control and storage function. The DC voltage is the source of energy for the chip.
The responder can varying dimensions from credit card size (the so-called microwave smartcard to dimensions of 0.5 by 0.25 m, depending on the use contemplated, for instance the identification of or electronic data interchange (EDI) with for instance persons, containers, cars, trains, and on the frequency band chosen. The frequency band can be in the range between about 1 GHz and about 40 GHz.
In operation, the responder receives signals from the interrogation unit, which signals are subsequently reflected or radiated again after having been provided with a code which unambiguously identifies the object or the person to which/whom this responder is attached or by which/whom it is carried otherwise. The responder can also be provided with new information.
The responder preferably draws the necessary energy entirely from the interrogation signal provided. Naturally, if the conditions are such that this energy supply is insufficient, use can be made of a battery to be arranged in the responder.
The antenna of the responder receives signals of a very low power density (typically smaller than 0.1 watt per square meter), modulates these signals, converts a part of the energy received into a DC voltage which provides the supply energy for a chip with a control and storage function, and reflects or radiates these signals again in the direction of the transmitter/receiver, also referred to as reader or interrogation unit, so as to transmit the information contained in the responder to the reader or interrogation unit.
Microwave responders are especially used for identifying very large numbers of objects or persons. The philosophy behind such responders must accordingly be "fit and forget", transponders without batteries being particularly qualified for use.
In order that sufficient DC voltage can be generated across a wide range of angles of incidence of the interrogation signal relative the responder and a largest possible reflection surface can be presented in the direction of the interrogation unit, it is desirable to choose the antenna amplification of the responder to be as large as possible.
A drawback of this choice is that a narrow beam width is inherent to a high antenna amplification. This means that there is less time available for providing the responder with energy and for reading out the responder and writing information in the transponder, and the available time may even become unacceptably reduced. This, in turn, means that this type of responder systems would be less eligible for use in situations where the speed of the objects to be identified or the angle of incidence of the microwave signal plays an important role.